This invention relates to an improved apparatus for withdrawing agglomerated solids from a fluidizing chamber and more particularly to an apparatus of the nature described wherein agglomeration is effected by introduction of a high velocity gas stream at a controlled velocity into a fluidized bed.
A coal gasification reactor of the type wherein agglomerated coal ash is withdrawn from a fluid reaction bed of finely divided coal without the removal of the finely divided coal particles is disclosed in Jequier et al, U.S. Pat. No. 2,906,608 and Matthews et al, U.S. Pat. No. 3,935,825. These patents are incorporated herewith by reference. In a coal to gas conversion process of the type referenced, a vessel is provided for a fluidized bed. A gas distribution grid is usually positioned in the vessel and defines the bottom surface of the fluidized bed. The central portion of the grid may be conical or cylindrical in shape and comprises a passage. At the bottom of the passage, a constriction is provided having a fixed opening defining a venturi of fixed throat size to provide a uniform upward gas velocity into the vessel and thus into the fluidized bed. Directing a stream of high velocity gas through the venturi or passage into the reaction vessel causes ash particles in the vessel to agglomerate and eventually discharge through the passage and venturi throat.
For a venturi of a given throat size, the gas velocity into the vessel is fixed unless the quantity of gases entering the venturi is varied. The degree to which gas velocity may be varied is limited because of several restrictions on the distribution of total gases entering the gasifier through the distribution grid and the venturi. However, variation in gas velocity is desirable to control the size of ash agglomerates removed from the fluidized bed. Also, the required amount of fluidizing agents (steam and air or oxygen) may change depending upon small variations in the properties of the feed coal, changes in ambient conditions and changes in coal moisture content. Changes in ash characteristics of the coal may also alter agglomerating tendencies of the ash and therefor require different quantities of air and oxygen to adjust the temperature in the agglomerating zone within the vessel. For example, to avoid a discharge of fines along with ash agglomerate as a result of a higher load of fines in the feed batch or an increase in the fluidized bed height, the velocity at the venturi or passage to the vessel would necessarily be increased.
Additionally, a coal gasifier should have a variable gas output and a capability for turndown. Thus, during a turndown period the total gas throughput to the gasifier would decrease. To maintain ash agglomeration and high carbon utilization, the gas velocity through the venturi would necessarily be maintained at the high values required under normal operation.
In a venturi having a cross section of a constant value, the only means for maintaining a fixed gas velocity at the venturi throat is to change the relative distribution of gases entering the gasifier vessel through the distribution grid and the inlet passage including the venturi. Unfortunately, two requirements, (1) the minimum superficial gas velocity across the distribution grid to avoid sinter formation, and (2) the different quantity of oxygen in the gas stream to the grid and the input passage, place severe restrictions on the limits to which the rate of flow of gases to the grid and the venturi passage can be varied. It is with this background that the present invention was conceived.